1. Technical Field
The present invention relates to a high strength and low yield ratio cold rolled steel sheet (including a plated steel sheet) having high elongation property and flange drawing property, and a method of manufacturing the same. More particularly, it relates to a high strength cold rolled steel sheet that has high tensile strength (TS) of 980 MPa or higher, high elongation property and flange drawing property such that [elongation property (El)×flange drawing property (λ)/yield ratio (%)] is 645 or higher, and a low yield ratio, a plated steel sheet made by plating the high strength cold rolled steel sheet, and a method of manufacturing the same.
The steel sheet of the present invention can be utilized in wide fields of industry including automobile, electric apparatuses and machinery. Description that follows will deal with a case of using the steel sheet of the present invention in the manufacture of automobile bodies, as a typical application.
2. Background Art
There are increasing demands for high-strength steel sheets for the purpose of improving the fuel efficiency through weight reduction of the steel sheets used in automobiles and improving the safety in the event of collision. Recently, calls for the reduction of exhaust gas emission based on concerns about the global environment add to the demands.
However, high-strength steel sheets are still required to have high workability for forming, so as to be formed in various shapes in accordance to the application. In an application where the steel sheet is pressed into a complicated shape, in particular, there is a strong demand for a high-strength steel sheet that combines satisfactory elongation property and flange drawing property.
A high-strength steel sheet developed to meet such needs is described in Japanese Unexamined Patent Publication (Kokai) No. 2003-89843 which discloses such a technology that improves elongation property and flange drawing property at the same time by forming the matrix phase structure substantially constituted from single phase of ferrite where precipitates containing V and Mo are dispersed. However, this technology is intended for the manufacture of steel sheets having tensile strength in a range from 600 to 750 MPa, and does not aim at the improvement of elongation property and flange drawing property in high strength region above 980 MPa.
A high-strength steel sheet known to have high ductility is residual austenite steel sheet made by forming residual austenite (γR) in the structure and causing induced transformation of γR (strain-induced transformation: TRIP) during forming step thereby improving the ductility.
For example, Japanese Unexamined Patent Publication (Kokai) No. 5-331591 discloses that satisfactory strength-ductility balance and low yield ratio can be achieved by forming the matrix phase structure from a mixture of ferrite containing the precipitation of ε-Cu and martensite or a mixture of martensite and residual austenite. Although this technology achieves improvements in elongation and yield ratio in high strength region above 980 MPa, it does not achieve sufficient flange drawing property and strength-ductility balance.
Japanese Unexamined Patent Publication (Kokai) No. 2001-140035 discloses that high ductility and high flange drawing property can be achieved by forming a composite structure containing ferrite in proportion of 30% or more in a volume ratio, residual austenite of 2% or more and low-temperature transformation phase (non-tempered martensite or bainite) in the steel sheet after annealing, while making ferrite gains finer. However, this technology is not intended for steel sheets in high strength region above 980 MPa, and addresses tensile strength in a range from 600 to 700 MPa.
Japanese Unexamined Patent Publication (Kokai) No. 2003-321738 describes that difference in hardness between soft ferrite phase and hard phase can be reduced so as to improve the flange drawing property without causing a decrease in ductility due to ferrite, by forming the matrix from a composite structure constituted from three phases of ferrite, bainite and residual austenite or four phases containing martensite in addition to the three phases, and causing dispersed precipitation of carbide containing Ti and Mo satisfying a formula. Japanese Unexamined Patent Publication (Kokai) No. 2000-282175 describes that crack initiating points can be reduced during a forming step thereby to achieve better strength-ductility balance and a low yield ratio without decreasing the strength, by forming a structure consisting of a principal phase constituted from bainite in a volume ratio from 60 to 90% and a second phase constituted from at least one kind of pearlite, ferrite, residual austenite and martensite.
However, technologies disclosed in Japanese Unexamined Patent Publication (Kokai) No. 2003-321738 and Japanese Unexamined Patent Publication (Kokai) No. 2000-282175 are related to hot-rolled steel sheets where the carbide mentioned above is precipitated during take-up step, and it is difficult to implement such technology in the manufacture of cold-rolled steel sheet. Elongation property and flange drawing property become lower as the sheet thickness decreases, and it is difficult to achieve the levels of elongation property and flange drawing property comparable to those of the hot-rolled steel sheet, in a cold rolled steel sheet that is usually thinner than the hot-rolled steel sheet.
The present inventors also have been conducting a research aimed at improving the elongation property and the flange drawing property of high strength cold rolled steel sheet. Accordingly, the present inventors proposed a steel sheet having matrix phase containing tempered martensite in a volume ratio of 15% or higher to the entire structure containing ferrite, and a second phase containing residual austenite in a volume ratio of 5 to 30% to the entire structure containing 0.8% or more C (for example, Japanese Unexamined Patent Publication (Kokai) No. 2003-171735). However, further improvements are required in order to improve the elongation property and the flange drawing property and reduce the yield ratio in steel sheets of higher strength.